Short-distance positioning system

ABSTRACT

The invention relates to a short distance positioning system for a mobile object with respect to a fixed reference system, in which:  
     the mobile object is equipped with a probe ( 1 ) comprising at least one resonant circuit (L 1 C 1 ) enabling reception of a magnetic field with frequency f g  and transmission of a magnetic field at frequency f e ; and,  
     the reference system ( 2 ) is provided with:  
     means of transmission/reception ( 3; 5 ) of a magnetic field comprising a voltage generator (G) generating a signal with frequency f g  powering at least one electromagnetic coil (L 3 ; L 5 ) and at least one capacitor (C 3 ; C 5 ) forming a series resonant circuit, with the said coil;  
     anti-dazzle means by which the transmission/reception means can distinguish the magnetic field with frequency f e  from the magnetic field with frequency f g ; and  
     detection and comparison ( 4 ) means to detect the amplitude of the voltage induced at the terminals of the resonant transmission/reception circuit and to deduce the displacement of the probe with respect to the reference system.

DOMAIN OF THE INVENTION

[0001] The invention relates to an electromagnetic positioning systemfor an object free to move with respect to a fixed reference system inwhich the displacement of the object is of the order of a fewcentimeters, with a precision better than 1 millimeter.

[0002] The invention is used for applications in many domains, andparticularly in the structural mechanics domain to study deformations ofsolids subject to stresses, or for protection to monitor the position ofan object that could be stolen or in the virtual reality domain toposition elements with a man-machine interface.

STATE OF THE ART

[0003] At the present time, there are many systems for positioning amobile object with respect to a fixed reference system. These includesome systems that are dedicated solely to short distance positioning, inother words displacement of the object that moves only a short distancewith respect to the reference system, in other words of the order of afew centimeters to a few meters.

[0004] One of these short distance positioning systems is a visualtechnique that consists of fixing a recognizable target on the mobileobject and aiming at this target with several cameras. Images from thecameras are then processed to determine the position of the target andconsequently the position of the object. However, these systems arelimited by the fact that the target has to be seen by the cameras, inother words it must be within the field of vision of these cameras.Therefore, no obstacles can be accepted. Furthermore, this system has alarge external infrastructure, in other words it is large; therefore, itis difficult to use it for positioning within a few centimeters.

[0005] A variant of this visual technique consists of using telemeters,for example laser telemeters that aim at the target. This technique isused to position an object moving over distances of a few centimeters;but it always has the disadvantage that the target must be visible fromthe telemeter.

[0006] A mechanical technique can also be used to position an object atshort distance, this technique consisting of fixing the object to a setof mechanical links. In this case, the link between the object to bepositioned and the reference system is mechanical, which hasdisadvantages in the sense that not all displacements may be authorized,and detection of displacement is not very precise.

[0007] Another technique for short distance positioning consists ofusing magnetic dipoles associated with magnetometers. Patent applicationEP-0-215 695 describes a system of this type. Positioning of magneticdipoles by magnetometers is adapted to positioning of objects movingover distances varying from a few centimeters up to a few meters; thesystem needs to be miniaturized to detect displacements of a fewcentimeters, in other words for very short distance positioning.However, it is difficult to miniaturize magnetometric sensors for whichthe precision must be high.

[0008] Furthermore, positioning an object in three dimensions makes useof complex calculation algorithms; therefore it is difficult to envisagethe use of this type of system to position an object moving within a fewcentimeters in three dimensions.

PRESENTATION OF THE INVENTION

[0009] The purpose of the invention is to overcome the disadvantages ofthe techniques described above. Consequently, it proposes anelectromagnetic system for positioning a mobile object with respect to afixed reference system, in which the displacement of the object is ofthe order of a few centimeters and the precision is better than 1millimeter.

[0010] More precisely, the invention relates to a system for positioninga mobile object with respect to a fixed reference system, characterizedby the fact that:

[0011] the mobile object is equipped with a probe (1) comprising atleast one resonant circuit enabling reception of a magnetic field withfrequency f_(g) and transmission of a magnetic field at frequency f_(e);and,

[0012] the reference system (2) is provided with:

[0013] means of transmission/reception of a magnetic field comprising avoltage generator (G) generating a signal with frequency f_(g) poweringat least one electromagnetic coil (L3; L5) and at least one capacitor(C3; C5) forming a series resonant circuit, with the said coil;

[0014] anti-dazzle means by which the transmission/reception means candistinguish the magnetic field with frequency f_(e) from the magneticfield with frequency f_(g); and

[0015] detection and comparison (4) means to detect the amplitude of thevoltage induced at the terminals of the resonant transmission/receptioncircuit and to deduce the displacement of the probe with respect to thereference system.

[0016] In a first embodiment, the transmission frequency f_(e) of themagnetic field and the reception frequency f_(g) of the magnetic fieldare equal, the probe comprising a single parallel resonant LC circuit.In this case, the magnetic field transmission/reception means mayinclude resistances connected to the resonant transmission/receptioncircuits and to the detection and comparison means, using a bridge setup.

[0017] In another embodiment, the probe comprises a first parallel LCresonant circuit for reception of the magnetic field with frequencyf_(g) and a second series LC resonant circuit for transmission of themagnetic field with frequency f_(e), where f_(g)≠f_(e).

[0018] According to another embodiment of the invention, the positioningsystem is characterized in that the transmission/reception meanscomprise:

[0019] a resonant transmission circuit for the magnetic field atfrequency f_(g); and

[0020] at least one reception circuit for the magnetic field atfrequency fe comprising a secondary coil connected to a capacitor toform a secondary resonant circuit.

[0021] Advantageously, the axes of the secondary coils are parallel toeach other and parallel to the axis of the probe coil.

[0022] The detection and comparison means may comprise:

[0023] a differential amplifier connected to the terminals of eachresonant reception circuit;

[0024] synchronization means for synchronizing signals output from eachdifferential amplifier; and

[0025] processing means to deduce the position of the probe startingfrom signals originating from secondary resonant circuits.

[0026] According to a first variant, the anti-dazzle means areelectromagnetic coils placed close to each secondary coil and powered bythe generator.

[0027] According to a second variant, the anti-dazzle means aredifferential amplifiers into which the signal output from the detectionand comparison means is applied to one input and an adjustable voltageis applied to another input.

BRIEF DESCRIPTION OF THE FIGURES

[0028]FIG. 1 diagrammatically shows the first embodiment of theelectromagnetic circuit of the system according to the invention;

[0029]FIGS. 2A and 2B diagrammatically show the second embodiment of theelectromagnetic circuit of the system according to the invention; and

[0030]FIG. 3 shows the second embodiment of the probe used in the systemaccording to the invention.

Detailed description of embodiments of the invention

[0031] This invention relates to an electromagnetic positioning systemcapable of positioning a mobile element that may or may not be visible,with respect to a fixed reference system, for a displacement of theorder of 1 centimeter.

[0032] In the system according to the invention, the mobile object to bepositioned is equipped with a probe made using at least one LC resonantcircuit, for the reception of a magnetic field at a frequency fg and thetransmission of a magnetic field at a frequency fe between about 100 kHzand 1 MHz.

[0033] According to a first embodiment of the probe, the frequenciesf_(g) and f_(e) are equal. In this mode, the probe comprises a singleresonant circuit formed from a coil L1 and a capacitor C1 connected inparallel. In this case, the voltage induced by the received magneticfield enables circulation of a current in the resonant circuit thatpasses through the induction coil L1 and generates a magnetic field.

[0034] This first embodiment of the probe is illustrated in FIG. 1 whichwill be described in detail later.

[0035] In a second embodiment shown in FIG. 3, the probe comprises twoLC resonant circuits. One of these resonant circuits comprises a coilL1′ and a capacitor C1′ connected in parallel; this circuit receives themagnetic field at frequency f_(g). The other circuit comprises a coil L2and a capacitor C2 connected in series; this circuit transmits amagnetic field with frequency f_(e)≠f_(g). In the example shown in FIG.3, the reception circuit L1′ C1′ and the transmission circuit L2 C2 areconnected to each other through a frequency doubler DF, for example madeby a diode bridge. In this case f_(e)=2×f_(g).

[0036] In order to simplify the description, the remainder of the systemaccording to the invention will be described for a probe conform withthe first embodiment, in other words with a single LC circuit.

[0037] In the system according to the invention, the fixed referencesystem is equipped with means of transmission of a magnetic field. Thesetransmission means consist of an LC resonant circuit and a voltagegenerator generating a signal at frequency f_(g). The LC resonantcircuit in these transmission means comprises an electromagnetic coilthat generates a magnetic field B at frequency f_(g). This circuit isresonant at a frequency f_(g), in other words the same frequency as thegenerator.

[0038]FIG. 1 diagrammatically shows a first embodiment of the systemaccording to the invention.

[0039] In this embodiment, the probe of the mobile object reference 1 isa probe with a single resonant circuit.

[0040] The fixed reference system is reference 2; it is equipped with anelectromagnetic circuit that comprises firstly means 3 oftransmission/reception of a magnetic field, and secondly means ofdetecting and comparing the two signals sampled at the terminals of thetransmission/reception means.

[0041] In this embodiment, the same circuit controls the transmission ofa magnetic field f_(g) and reception of the magnetic field f_(e) (wheref_(g)=f_(e))

[0042] The transmission/reception means 3 comprise a generator Gtransmitting a signal at frequency f_(g) and a resonant circuit composedof a coil L3 and a capacitor C3 connected in series.

[0043] The function of this coil L3 is firstly to transmit a magneticfield at the same frequency f_(e) as the resonant frequency of theprobe, and to be sensitive to the position of the probe.

[0044] The transmission/reception means 3 also comprise two resistancesR1 and R2 in series installed with the resonant circuit L3C3 and thevariable resistance R_(v) to form a bridge. Detection and comparisonmeans 4 are connected between the two legs of the transmission/receptionmeans, in other words firstly between resistances R1 and R2, andsecondly at the output from the L3C3 circuit to compare the voltages ofthe signals circulating in these two legs.

[0045] These detection and comparison means may consist of adifferential amplifier reference A, for which the + and − inputterminals are connected to the bridge assembly legs. The output of theamplifier A is connected to synchronous detection means D, themselvesconnected to generator G. Thus, the voltage at the terminals of thedifferential amplifier A is detected by synchronous detection withgenerator G.

[0046] In this embodiment of the invention, the antidazzle means aremade by the measurement bridge (circuit L3C3+resistances R1, R2, R_(v))The function of these anti-dazzle means at the time of reception of themagnetic field with frequency f_(e), is to enable better recognition ofthe said magnetic field by the transmission/reception means. In otherwords, amplifier A is not dazzled by the transmission field withfrequency f_(g). Furthermore, these anti-dazzle means enable thedetection means to not detect the signal originating from thetransmission through the fixed reference system.

[0047] In FIG. 1, the magnetic field transmitted by coil L3 and receivedby the resonant circuit of probe 1 is marked as reference B. The coil L3is associated with a capacitor C3 such that the resonant frequency ofcircuit L3C3 is equal to the generator frequency f_(g.) At resonance,the impedance of this circuit L3C3 is limited to the value of the lossresistance Rs of the coil. This entire circuit forms part of a bridgeset up. The impedance denoted Z placed in series with the coil L3represents a term due to coupling between L3 and L1, matched at the samefrequency as the generator. This impedance Z depends only on theproperties of resonant circuits L3C3 and L1C1, and their geometricposition with respect to each other.

[0048] When the probe is at a very large distance compared with the coildimensions, the impedance Z is zero. The condition for balancing thebridge is then written RsR2=R1Rv, where Rv is a variable resistance usedto balance the bridge. When the probe is brought towards the referencesystem 2, a non-zero impedance value Z appears which induces a voltagedifference at the terminals of the comparison means 4.

[0049] Thus, it can be understood that the system can be used toestimate variations of the distance between the probe 1 and the fixedreference system 2 in which the coil L3 is located.

[0050] In practice, it is useful to adjust the equilibrium of the bridgeat a rest distance r and therefore only observe non-zero voltage forvariations of the distance between the probe and the reference system.In other words, if variations in the distance d between the probe andthe reference system are small compared with the rest distance r, thedetected voltage can be obtained analytically by relations known to anexpert in the subject, such as those described in F. TERMAN, RadioEngineer's handbook, McGraw Hill Inc., New York and London, 1943, pp.67-73 and 148-164.

[0051] Another method of adjusting the system is simply to calibrate it.

[0052] Note that if this system is sensitive to variations in thedistance d, it is also sensitive to variations in the orientation of theaxes of the coils with respect to each other. Thus, it is preferablethat the axes of coils L3 and L1 are parallel, in other words that theyhave the same direction and sign.

[0053] FIGS. 2A and 2B show another embodiment of the system accordingto the invention.

[0054] In this embodiment, the bridge set up of the electromagneticcircuit of reference system 2 is replaced by a signal compensationcircuit made using counter coils.

[0055] In this embodiment, the probe of the mobile object is chosen tobe identical to the probe for the first embodiment. Therefore, itsreferences are identical to those in FIG. 1.

[0056] As shown in FIG. 2A, the reference system 2 in this embodimentcomprises a resonant transmission circuit reference 5, and severalresonant reception circuits references 6, 7 and 8. The resonantreception circuits each comprise a reception coil L6, L7 or L8 connectedin parallel with a capacitor C6, C7 or C8 respectively.

[0057] As in the first embodiment, the function of the transmission coilL5 of the transmission means 5 is to generate a magnetic field B atfrequency f_(g).

[0058] Therefore, the probe is subjected to a magnetic field thatinduces a current in its LC circuit. The probe then behaves like asecondary transmission circuit and in turn generates a magnetic fieldthat is input to reception circuits 6, 7 and 8.

[0059] The fact that several reception circuits are used is a means ofincreasing the number of degrees of freedom in the displacement of theobject. Thus, the invention proposes to use three reception circuits inorder to detect a displacement in three dimensions.

[0060] As already explained, the axis of coil L5 is preferably parallelto the axis of coil L1 of the probe.

[0061] Similarly, the axes of the secondary coils are preferablyparallel to the axis of coil L1 of the probe.

[0062] As shown in FIGS. 2A and 2B, each resonant circuit 5, 6, 7 and 8is connected to an electronic circuit for processing this information.More precisely, the electromagnetic circuit for reference system 2comprises pins E1 and E2 making the connection between the generator Gand the resonant circuit L5C5. For example, the generator could be afrequency oscillator f_(g).

[0063] In order to compensate for the reactance of coil L5 at thegenerator frequency f_(g), a capacitor CS is connected in series withthis coil L5 such that 4π²L5C5 =1. The generator G will then only seethe real impedance of the losses of coil L5. The magnetic field Bcreated at the rest distance r5 between coil L5 and probe 1 is thenwritten in the following form:${B_{0} = \frac{\mu_{0}M}{4\pi \quad {r5}^{3}}},$

[0064] where μ₀ is the permittivity of a vacuum and M is the dipolemoment of the transmission coil L5, where M=niS, n is the number ofturns in the transmission coil, S is its cross-section and i is thecurrent that passes through it at frequency f_(g).

[0065] The current induced in the probe is then:${i_{0} = {\frac{2\pi \quad f_{g}}{R\quad s}B_{0}}},$

[0066] where Rs is the series resistance of the coil L5 (Rs is not shownin this figure in order to simplify the figure).

[0067] At the rest distance r6 between the reception circuit 6 and theprobe 1, the secondary magnetic field B created by probe 1 is in thefollowing form: $B_{6} = \frac{\mu_{0}M}{4\pi \quad {r6}^{3}}$

[0068] The same is true for secondary magnetic fields B₇ and B₈ withrespect to distances r7 and r8.

[0069] Each of these magnetic fields B₆, B₇ or B₈ creates a voltageinduced in the corresponding coil L6, L7 or L8 with amplitudes27πf_(g)B₆, 2πf_(g)B₇, 2πf_(g)B₈, respectively. This voltage ismultiplied by the overvoltage coefficient of the corresponding resonantcircuit L6 C6, L7C7 or L8C8, provided that the said circuit is matchedat frequency f_(g).

[0070] As shown in FIG. 2B, the reception means 6 are connected to anamplifier A6 by means of pins R61 and R62 placed at each end of the coilL6. The amplitude of the signal output from the reception means andamplified by A6 is detected by comparison with the signal generated bythe generator G and shifted appropriately by the phase shifter DP. Forexample, this comparison may be made by a synchronous detector D6controlled by the oscillator G and the phase shifter DP. Thissynchronous detection is used to filter the signal originating from thereception means on a narrow band and therefore to limit the sensitivityof the system to parasites.

[0071] However, note that these detection means are only necessary whenthe transmission frequency f_(g) and the reception frequency f_(r) areequal. If not (for example if the probe in FIG. 3 is used), thesedetection means are not useful. In this case, it is sufficient to filterthe signal output from the amplifier around frequency f_(e).

[0072] The electronic processing means of the signals originating fromthe reception means 7 and 8 are identical to the electronic processingmeans that have just been described for signals output from receptionmeans 6.

[0073] The system according to the invention comprises anti-dazzle meansthat prevent the secondary coils L6, L7 and L8 from being “blinded” bythe magnetic field originating from the coil L5, in other words suchthat coils L6, L7 and L8 react only to the magnetic field with frequencyf_(e) retransmitted by probe 1.

[0074] For example, these anti-dazzle means may consist of additionalcoils that are smaller than coils L6, L7 and L8; in this case, thecurrent input to the transmission coil L5 also powers these additionalcoils placed close to each of the secondary coils L6, L7 and L8, and thefunction of which is to create a field at the secondary coils equal toand in the opposite direction from the field transmitted by thetransmission coil L5. Thus, when there are no mobile objects close tothe reference system, the field induced in the secondary coils is zero.

[0075] These anti-dazzle means can also be made electronically by meansof differential amplifiers. In this case, a differential amplifier Difis connected to the output of each detection device D6, D7 and D8; moreprecisely, the signal S6, S7 or S8 output from the synchronous detectorsD6, D7 or D8 is input to the +terminal of each amplifier Dif, and thevariable voltage V_(ref) is input to the−terminal. This voltage V_(ref)is adjusted such that it cancels the signals S6, S7 or S8 when there isno probe.

[0076] One variant consists of combining the two anti-dazzle meansdescribed above; in this case, the electronic anti-dazzle means can beused to recover anti-dazzle defects, for example due to geometricimperfections.

[0077] Outputs S6, S7 and S8 from the synchronous detectors [or outputsS′6, S′7 or S′8 if differential amplifiers Dif are used], are connectedto processing means T that use these signals to determine the exactposition of the probe with respect to the reference system. Variationsin the position of probe 1 with respect to the reference system 2 inducevoltage variations at the outputs from the three detection systems S6,S7 and S8. It is possible to correlate these variations with restdistances r6, r7 and r8 using the equations given previously, and thusto determine the position of the probe, provided that the correspondingpositions of the transmission means and the three reception means 6, 7and 8 are known, taking account of the fact that two distances r5 and r6(or r7 and r8, or . . . ) form a triangle.

[0078] These processing means T are conventional calculation meansderived from field equations or obtained by learning.

[0079] The invention was described above in a configuration in whichthere is one transmission means 5 and three reception means 6, 7 and 8,and in which the axis of the probe is parallel to the axes of the othercoils. However, it is possible that the axis of the probe is notparallel to the axes of the other coils, and in this case an additionalvariable is used to increase the number of equations to correspond tothe number of unknowns, this variable being related to the use ofadditional reception means with a perpendicular axis.

[0080] Advantageously, the electronic circuit in FIG. 2B may beminiaturized in the form of a specific integrated circuit; this circuitmay be integrated into the support containing the coils of transmissionand reception circuits in FIG. 2A. In other words, for example, thesystem according to the invention may be made on a printed circuit withcoils printed in the form of a spiral. The electronic circuit in FIG. 2Bis then implemented on the same circuit, either in the form of surfacemounted integrated components, or in the form of a specific circuit.Similarly, the probe may be made on a printed circuit.

[0081] The system according to the invention may also be made usingmicrocoils in order to make the system very compact, in other words withdimensions of the same order of magnitude as the positioning distance,namely of the order of 1 centimeter or less. Similarly, the dimensionsof the probe may be extremely small, for example of the order of 1millimeter.

1. Positioning system for a mobile object with respect to a fixedreference system, characterized by the fact that: the mobile object isequipped with a probe (1) comprising at least one resonant circuit(L1C1) enabling reception of a magnetic field with frequency f_(g) andtransmission of a magnetic field at frequency f_(e); and, the referencesystem (2) is provided with: means of transmission/reception (3; 5 ) ofa magnetic field comprising a voltage generator (G) generating a signalwith frequency f_(g) powering at least one electromagnetic coil (L3; L5)and at least one capacitor (C3; C5) forming a series resonant circuit,with the said coil; anti-dazzle means by which thetransmission/reception means can distinguish the magnetic field withfrequency fe from the magnetic field with frequency f_(g); and detectionand comparison (4) means to detect the amplitude of the voltage inducedat the terminals of the resonant transmission/reception circuit and todeduce the displacement of the probe with respect to the referencesystem.
 2. Positioning system according to claim 1, characterized inthat the transmission frequency f_(e) Of the magnetic field and thereception frequency f_(g) of the magnetic field are equal, the probecomprising a single parallel resonant LC circuit.
 3. Positioning systemaccording to claim 2, characterized in that the magnetic fieldtransmission/reception means include resistances (R1, R2, Rv) connectedto the resonant transmission/reception circuit and to the comparisonmeans, using a bridge set up.
 4. Positioning system according to claim1, characterized in that the probe comprises a first parallel LCresonant circuit for reception of the magnetic field with frequencyf_(g) and a second LC resonant circuit in series for transmission of themagnetic field with frequency f_(e), where f_(g)≠ f_(e.)
 5. Positioningsystem according to any one of claims 1, 2 or 4, characterized in thatthe transmission/reception means comprise: a resonant transmissioncircuit (5) for the magnetic field at frequency f_(g); and at least onereception circuit (6, 7, 8) for the magnetic field at frequency f_(e)comprising a secondary coil (L6, L7, L8) connected to a capacitor (C6,C7, C8) to form a secondary resonant circuit.
 6. Positioning systemaccording to any one of claims 1 to 5, characterized in that the axes ofthe coils are parallel to each other.
 7. Positioning system according toany one of claims 1 to 6, characterized in that the detection andcomparison means (4) comprise: a differential amplifier (A; A6, A7, A8)connected to the terminals of each resonant reception circuit;synchronization means (D; D6, D7, D8) to synchronize signals output fromeach differential amplifier with the generator signal; and processingmeans (T) to deduce the position of the probe from signals originatingfrom secondary resonant circuits.
 8. System according to any one ofclaims 5 to 7, characterized in that the anti-dazzle means areelectromagnetic coils (L′) placed close to each secondary coil andpowered by the generator.
 9. System according to any one of claims 5 to7, characterized in that the anti-dazzle means are differentialamplifiers into which the signal output from the detection andcomparison means is input and an adjustable voltage is applied toanother input.